1. Field of the Invention
The invention relates to the structure for a rod radiation source and its calibration phantom. Especially, it is the calibration phantom formed in a cylindrical container by utilizing a rod radiation source with different sheet materials. It is used to obtain the calibration curve for density and counting efficiency and measure the gamma radiation activity for different nuclear wastes.
2. Description of the Prior Art
Presently, the total gamma radiation activity measurement equipment for low-activity bulk nuclear wastes is the Waste Curie Monitor assembled from large-area plastic scintillation detectors. Its advantages are high radiation sensitivity, high counting efficiency, no limit on waste volume and capability of indicating sample weight etc.
However, the traditional Waste Curie Monitor has the following drawbacks for its calibration method:
1. Underestimate or overestimate for activity measurement: it usually considers weight only, and therefore it overlooks the self-absorption effect due to the different material density for different waste sample.
2. Unsuitability for counting efficiency calibration: it usually uses a single material in a monitor shield to establish density efficiency, but overlooks the compositions for non-single materials.
3. Incompliance for sample measurement location: it usually puts the sample in the lower place in the shield and therefore the location deviates from geometric center for efficiency calibration.
4. Incompliance for sample volume: it usually does not restrict sample volume; the inconsistent distance from each scintillation detector during efficiency calibration causes errors in activity analysis; thus, it can not meet the accuracy requirement for radioactive waste activity analysis by radiation protection safety management.
5. There is no correction for the total activity measurement error in radiation energy and gamma activity calculation for the waste samples with various radioactive nuclides.
In recent years, researchers further develop new methods to replace the traditional calibration method that uses a point radiation source at the geometric center of the activity monitor. They are used for total gamma activity measurement for radioactive wastes in the classification process to identify and distinguish the radioactive wastes from non-radioactive wastes. The efficiency calibration method for present plastic scintillation detector is described in the following:
1. US Themo-Eberline company uses transmission factor to correct for the self-absorption effect due to different standard mass. The formula is TF=net count for shielded radiation source/net count for non-shielded radiation source. In general, TF≦1.0 as the calibration efficiency at geometric center for non-shielded radiation source in compensative air. The transmission factor for the water phantom that has completed efficiency calibration is set to 1. Sample weight is entered for mass parameter (one unit per 10 kg). Therefore, the sample with different weight will obtain corrected total gamma activity against the weight of standard that has transmission factor 1.
2. German RADOS company uses multi-density calibration efficiency from a single material of iron sheet.
3. Japan Nuclear Energy Safety Organization uses multi-nuclide calibration efficiency from metal pipes and metal sheets.
4. US NE Technology company uses multi-weight (o˜60 Kg) calibration efficiency from multi-nuclide point radiation source and single material Brazil wood (density=1).
Nevertheless, present correction methods only consider approximate weight and geometry, and therefore do not provide full correction for self-absorption effect due to material mass and interactions between various material masses and energy. They all fail to provide accurate total gamma activity.
In view of the above drawbacks with the traditional radiation source calibration methods, the inventor has made significant improvement in the invention.